The present invention concerns a method for managing the functioning under irradiation of at least one electronic component with complementary MOS (metal-oxide-semiconductor) transistors. It also concerns a device powering such an electronic component in systems including one or more such devices.
Within the meaning of the present invention, electronic component with complementary MOS or CMOS transistors means not only individual elementary components such as gates or inverter gates including CMOS transistors, but also assemblies formed by a plurality of individual elementary components, such as circuits, parts of circuits, microprocessors or computers, including CMOS transistors.
The invention finds application in the manufacture of electronic devices, such as control devices, which can be used in a hostile atmosphere of ionising radiations, notably in the civil nuclear industry. The invention also finds application in the production of embedded electronic devices intended to be used in a hostile ionising radiation environment.
The development of electronic systems used in the civil nuclear industry has increased considerably during the 1990s.
For reasons of cost, delivery times and legislation, these specific components designed to resist radiation have, to a great extent, been replaced by very widespread standard components, in particular by components of the CMOS type (components with transistors of the complementary metal-oxide-semiconductor type).
Thus, by selecting in advance components whose resistance to radiation is good and by complying with certain design rules for the architecture of the systems, these systems can be used with ionising radiation doses greater than 1 Mrad.
Studies currently being carried out even show that increasing the quality of the methods of manufacturing, the intrinsic resistance of the components improves very significantly. It nevertheless remains true that the versatility of the new components makes it difficult to control in advance the resistance to radiation. With the majority of embedded applications, it is important to react preventatively to the loss of functionality of an electronic system. A loss of functionality can in fact be the cause of a significant degradation of the components due to the assimilated radiation dose.
It is considered that a component is subject to a loss of functionality either when it has failed or when the execution of the tasks assigned to it are not error-free.
If the architecture of an electronic system is designed with a certain number of redundant elements or parts, it is possible to reconfigure the system in order to connect up new elements which were not powered and to disconnect elements which were powered. In this regard reference can be made, for example, to Rad-hard Embedded Computer for Nuclear Robotics by A. Giraud et al, Conference Proceedings RADECS 1993, pages 43-47.
Such a reconfiguration thus makes it possible to eliminate from the system, in a preventive, temporary or definitive fashion, the components which have failed or are liable to cause a failure.
The reconfiguration of an electronic system subjected to radiation, during which some components are powered down, makes it possible to avoid excessive degradation of the components and even to xe2x80x9cregeneratexe2x80x9d the latter. It is in fact known that the electronic components of microcontrollers and particularly components of the MOS (metaloxide-semiconductor) type, subjected to ionising radiation, but powered down, may recover, at least partly, their initial characteristics, after having being degraded when they were powered down.
The phenomenon of regeneration of powered-down components, in the presence of radiation, is due to a discharge of the charges caused by the radiation and by an effect of compensation and redistribution of the charges. In particular, in MOS components, the holes migrate towards the oxide-semiconductor interface region in order to compensate for the charges trapped in the oxide layer. In this regard reference can be made to French Patent Applications FR-A-2 721 122 and FR-A-2 633 160.
In order to envisage a reconfiguration of an electronic system and anticipate any failures of the components, it is necessary to establish a relationship between the ionising radiation dose received by the components and the functionality of the system.
In order to determine the radiation dose received by the components, it is known that a shifting of the threshold voltage of the CMOS components can be taken into account.
By way of example, a simple CMOS component, such as an inverter gate of the 7404 type, can be used as a dosimeter. An inverter gate of the 7407 type is composed of a pair of NMOS and PMOS transistors. For a given supply voltage +Vcc for the component, the switching threshold of the inverter gate is around Vcc/2. When the component is subjected to radiation, this threshold decreases. Depending on the biasing conditions during the irradiation (high biasing) the threshold can even become negative.
It appears that, under nil biasing, during the irradiation of the component, the threshold voltage of the inverter gate is a relatively homogeneous function of the dose received.
Associated with an electronic system, an inverter gate of the 7404 type with MOS components can thus serve to measure the radiation dose received. In this regard reference can be made to the document Handbook of Radiation Effects, by Andrew Holmes-Siedle/Lens Adams, Oxford Science Publications, pages 110-113.
Another useful parameter for measuring the irradiation is the consumption current of the complementary MOS (CMOS) components. The consumption current of a CMOS component increases with the radiation dose received. This is a consequence of the threshold voltage of 0 volts of the NMOS (n-type MOS) transistors being exceeded. The leakage current of the transistors increases in fact when the threshold voltage is negative. Thus, in certain devices, the measurement of the quiescent current is used to monitor the ionising radiation dose received. In this regard reference can be made to Total-Dose Issues for Microelectronics in Space Systems, by Ronald L. Pease, IEEE Transactions on Nuclear Science, Vol. 43, No. 2, April 1996, pages 442-450.
It appears, however, that the use of the parameter of the current consumed by a component does not well represent the availability of the component or of the electronic system on which this current is measured. This is because the intensity of the current remains sensitive to the dose rate and does not show the influence of a regeneration of the component. It is normal to observe a rapid increase in the consumed current followed by a slower decrease, without the availability of the component or components being effected thereby. Moreover, the electronic components subjected to radiation are usually able to function beyond the characteristics supplied by the manufacture of these components.
The devices or methods for determining the irradiation doses described above certainly make it possible to indicate that predetermined critical thresholds have been exceeded but do not guarantee the functioning of an electronic system for a given task to be accomplished in a given length of time.
Thus, in order to increase the reliability of the functioning of an electronic system, this system is generally oversized. Such a measure, however, has negative consequences on the cost of the system, its complexity and its bulk.
In addition, the means described above do not take account of the phenomenon of regeneration of the components, already mentioned.
The aim of the invention is to propose a method and device for managing the functioning of electronic components which do not have the limitations set out above.
One aim of the invention is in fact to guarantee the functioning of a component or of a plurality of electronic components for a given length of time, taking account of the ionising radiation received by the components during this time.
Another aim of the invention is to take into account the ability of the components to regenerate themselves when they are not powered up.
To achieve these aims the object of the invention is more precisely a method of managing the functioning under irradiation of at least one electronic component having a nominal supply voltage Vnom, in which:
during a so-called test phase, there is applied to the component an initial supply voltage Vinit less than the nominal voltage Vnom and greater than or equal to a minimum operating voltage Vmin and a check is carried out on the functioning of the component, and
during a so-called working phase, initiated when the check has revealed correct functioning of the component, there is applied to the component a working supply voltage Vsupp greater than the initial supply voltage Vinit.
The invention applies in particular to components comprising one or more stages of complementary transistors of the MOS (metal-oxide-semiconductor) type.
The method can also include a so-called off-load phase, initiated either when the working phase is terminated or when the check has revealed defective functioning. During this phase, an off-load voltage is applied to the component.
Within the meaning of the invention, nominal supply voltage means the voltage at which the component should normally be supplied for its functioning in a device in which it is integrated.
In addition, the minimum operating voltage is defined as the lowest supply voltage necessary to the component to execute a task without loss of functionality.
The invention is based on the finding that the correct functioning of an irradiated electronic component can be ensured by applying a higher supply voltage to it.
In other words, an increase xcex94V in the supply voltage makes it possible to maintain the correct functioning of a component for a given irradiation dose assimilated by the component during a working phase. Thus, by measuring the ionising radiation dose assimilated by the component it is possible to determine a period of functioning or working phase period during which the correct functioning of the component, whose supply voltage is increased by xcex94V, can be maintained.
By virtue of the method of the invention, if the correct functioning of the component is verified for the initial voltage Vinit, functioning during the working phase at a voltage greater than the initial voltage by at least xcex94V, can be maintained.
In particular, the component can be supplied during the working phase with a supply voltage substantially equal to its nominal voltage.
In this case, the initial voltage at which the test is carried out is such that Vinit=Vnomxe2x88x92xcex94V.
In order to guarantee functioning under radiation for a given period, the period of the working phase can be adjusted as a function of the increase in the supply voltage xcex94V due to the radiation.
For a given dose assimilated by a component, the supply voltage of this component has a minimum operating value denoted Vmin.
Thus, according to a particular aspect of the invention, it is possible to determine, for each electronic component, the minimum operating voltage under irradiation Vmin and the duration of the working phase with respect to the duration of the off-load phase is adjusted as a function of the said minimum operating voltage.
More precisely, it is also possible to adjust the duration of the working phase and of the off-load phase as a function of a difference between the nominal voltage and the minimum voltage.
These measurements make it possible to obtain an optimum functioning of the component or components by taking into account their ability to be regenerated during the off-load phase when an off-load voltage, preferably zero, is applied to them. In addition, in order to perfectly take account of the regeneration of the components, it is possible to determine the minimum operating voltage during or just after the off-load phase.
According to another aspect of the invention, a method is defined in which:
a minimum operating voltage Vmin of the component under irradiation is determined, then
the component is supplied at a supply voltage Vsupp such that Vmin+xcex94Vxe2x89xa6Vsuppxe2x89xa6Vnom, during a working phase whose duration is determined according to xcex94V, xcex94V being a voltage, and then
an off-load voltage is applied to the component during an off-load phase.
Thus, as the supply voltage is greater than the minimum operating voltage by a quantity at least equal to xcex94V, the component can assimilate an irradiation dose, a function of xcex94V, without losing its functionality. The link between the voltage difference xcex94V and the permissible irradiation dose can be established experimentally.
Another object of the invention is a supply control device for at least one electronic component including:
means of controlling the functioning the electronic component,
a nominal supply voltage source,
an off-load voltage source,
a so-called initial voltage source, less than the nominal voltage, and
selection means controlled by the control means in order to selectively apply to the component:
the initial voltage during a control phase,
the nominal supply voltage during a working phase when the functioning of the component is correct during the control phase,
the off-load voltage during an off-load phase, when the functioning of the electronic component is defective during the control phase.
The selection means are for example an electronic gate with three inputs connected respectively to the nominal voltage source, to the off-load voltage source and to the initial voltage source.
The means of controlling the correct functioning of the component can be test circuits able to carry out tests such as, for example, xe2x80x9clifexe2x80x9d tests, xe2x80x9ccoherencexe2x80x9d tests or xe2x80x9cautotestsxe2x80x9d. The control means can thus include circuits of the xe2x80x9cwatchdogxe2x80x9d type.
Circuits of the xe2x80x9cwatchdogxe2x80x9d type are generally provided for components such as a microprocessor. The microprocessor must regularly apply to the xe2x80x9cwatchdogxe2x80x9d circuit a sign-of-life pulse which constitutes a check on its correct functioning.
The invention also concerns a computer comprising a plurality of redundant calculation units able to function in turn. In accordance with the invention, each calculation unit is equipped with a supply control device as described above.
In particular, each calculation unit can be a microcontroller.
In order to control the functioning of the calculation units in turn, such a microcontroller can be programmed so as to control, when a working phase is completed, the energising of another calculation unit.
More precisely, each calculation unit can be programmed to:
a) select a following calculation unit when a working phase is completed,
b) control the control means of the supply control device of the said following calculation unit in order to initiate a control phase and a working phase if the check reveals correct functioning,
c) select another following calculation unit if the check reveals defective functioning.
As a variant of the system described above, the invention also concerns an electronic system including a plurality of the calculation units and an electronic monitoring module for the functioning of the calculation units in turn. In this system, the monitoring module has at least one supply control device as described above, associated with the plurality of calculation units.
Another object of the invention is a method of testing a component comprising at least one transistor of the CMOS type, in which:
the component is caused to function whilst decreasing the supply voltage for the component until a defective functioning of the component is detected,
a reading is taken of the supply voltage (Vmin) of the component below which defective functioning occurs, and
this supply voltage is compared with a nominal supply voltage (Vnom) for the component in order to establish an operating margin of tolerance for the component under irradiation.
The operating margin of tolerance can be understood, for example, as the difference between the nominal voltage and the voltage below which effective functioning is observed. The operating margin of tolerance thus constitutes an item of information on the quality of the component, which expresses its suitability for functioning in an irradiated environment.
Other characteristics and advantages of the invention will emerge more clearly from the description which follows, with reference to the figures in the accompanying drawings, given purely for illustration and non-limitatively.